Visualization during minimally invasive surgical procedures has long been understood to enhance the performance and outcomes of surgical procedures. However, successful visualization, particularly visualization into a tissue volume, has proven elusive. One promising catheter-based visualization technology is optical coherence tomography (OCT). OCT has shown promise as an “ultrasound-like” optical visualization method, in which a thickness of the tissue volume may be imaged to reveal internal structures at relatively high resolution.
OCT may be particularly useful in conjunction with a catheter that may traverse tissues and body lumens and may, in some variations, be configured to modify or sample tissue in conjunction with the imaging or guided by the imaging. For example, an OCT imaging catheter may be configured as an atherectomy catheter. A significant body of scientific and clinical evidence supports atherectomy as a viable primary or adjunctive therapy prior to stenting for the treatment of occlusive coronary artery disease. Atherectomy offers a simple mechanical advantage over alternative therapies. By removing the majority of plaque mass (debulking) it creates a larger initial lumen and dramatically increases the compliance of the arterial wall. As a result, for example, stent deployment would be greatly enhanced following site preparation with atherectomy. There are advantages related to the arterial healing response. By removing the disease with minimal force applied to the vessel and reducing the plaque burden prior to stent placement, large gains in lumen size can be created with decreased vessel wall injury and limited elastic recoil. This has been shown to translate into better acute results and lower restenosis rates.
Physician practice is often to a treat target lesion as if it is composed of concentric disease even though intravascular diagnostic devices have consistently shown significantly eccentric lesions. This circumferential treatment approach virtually ensures that native arterial wall and potentially healthy vessel will be stressed, stretched or cut unnecessarily.
Currently available systems are poorly adapted for real-time imaging, particularly for use in catheters including atherectomy catheters. For example, much is already known about FORJ technology (Fiber Optic Rotating Junction), spinning mirrors, spinning prisms, and motors in the distal tips of catheters. However, such embodiments take up a lot of space, so much so that they may not be practical for use in conjunction with a therapeutic embodiment such as an atherectomy device.
It is generally desirable to reduce the crossing profile of the catheter to enable access to distal tortuous vessels in the heart or the periphery without collateral damage. The invention described here may achieve these aims. There are no large, expensive, fragile rotating junctions or rotating mechanisms in the catheter distal tip. The fiber is terminated in an adhesive that forms a single, unique, well-defined reference reflection with no complicating intermediate reflections. The drive shaft can have a small OD (0.012″ demonstrated), minimizing the effect on crossing profile.
The devices described herein may form a circumferential view using the imaging catheter, allowing a true full circumferential field of view with a very small impact on crossing profile while preserving the ability to use common-path interferometry. Prior art devices (e.g., Lightlab™ ImageWire, MGH fiber optic rotating junctions, Cardiospectra (Milner)) generate full circumferential views inside a body lumen either by having a fiber rotating junction (e.g., http://www.princetel.com/product_forj.asp) between the OCT console and the catheter tip, with spinning of the optical fiber, by having a mechanism on the end of the catheter that rotates a mirror or prism, or by wagging the fiber in one or two axes or planes.
A FORJ necessarily introduces a break in the fiber. In this type of system, light goes from being confined in the core of the fiber to propagating in free space and is then re-imaged back into the fiber. See, e.g., Bouma (U.S. Pat. No. 7,382,949). Two problems immediately ensue from this arrangement. First, the break in the fiber and the re-imaging optics create several surfaces with potentially very large return losses (back-reflections) compared to the usual OCT reference reflection. This makes the device difficult to use with common-path interferometry, as the interferometer will index off the first substantial reflection. One cannot simply make the reference reflection brighter than these surfaces, as (a) this would then create a reference reflection that could saturate the detector if it needed to be greater than, for example, 20 microWatts, and (b) the strong reflections present in the proximal optical path could still lead to artifacts in the OCT image, as these reflective surfaces would still be orders of magnitude brighter than the signal from the tissue. Second, the alignment of the two fiber cores has to be maintained to an exceptionally high tolerance, typically less than 0.5 microns of wobble as the device rotates. Such a high level of accuracy drives up the cost of the device significantly, which is something of particular concern in a single-use disposable device.
One attempted solution to the internal reflection problem in the FORJ is to have a rotating junction that incorporates index matching fluid between the fixed and rotating fiber cores. This solution is not really suitable for cost and complexity reasons as a component of a one-time-use disposable catheter. Incorporating the FORJ into the capital equipment complicates the design of the interface as this now has to be a sterilizable multi-use unit resistant to liquid and contaminant ingress. These requirements may be incompatible with the materials and assembly techniques used to make the FORJ.
Furthermore, a rotating mechanism on the distal tip significantly increases the crossing profile and complexity of the device. It is generally unsuitable for use with a single-use disposable device where costs must be minimized. In a device intended for small diameter body lumens, for example coronary arteries, the presence of a large diameter mechanism in the distal tip will define the maximum vessel size that can be safely treated. The mechanism may also increase the rigid length of the catheter, which will in turn restrict the vessel tortuosity into which the catheter may be safely inserted. This may preclude use of the device in the mid- or distal coronary arteries or in the distal peripheral vasculature, for example the dorsalis pedis.
The methods, devices and systems described herein allow intra-luminal common-path low-coherence interferometry with a contiguous fiber path while also allowing the creation of and updating of 360° circumferential views inside a vessel with angle and longitudinal encoding. Common-path interferometry is highly desirable for a catheter, as it eliminates the need for a separate reference arm and makes operation insensitive to manufacturing variations in catheter length. The devices, systems and methods described herein allow for creation of a >360° circumferential view as well as a 3-D reconstruction or rendition of the traversed tissue volume, without a break in fiber continuity. These methods, devices and system are particularly suitable for directional atherectomy or directional re-entry, as the imaging element can be steered towards a region of interest and allowed to dwell there so that the cut progression and depth can be monitored in real time.
There is a need for a method of forming a circumferential image in a lumen in a manner that permits the use of common-path interferometry and that has a minimal impact on crossing profile and work flow in the catheter lab. Common path interferometry eliminates the down-lead sensitivity that makes catheters for Michelson interferometry very costly to produce. This is because the catheter length has to be matched to the reference arm in the console to within a few microns or to within the adjustability of the reference arm. Common-path interferometry also allows the console to be placed an almost arbitrary distance from the patient and fluoroscopy equipment. The invention described here achieves these aims. The fiber is contiguous from console to distal tip, with no breaks to cause large back-reflections thereby permitting common path interferometry.
Furthermore, it would be very useful to provide catheter devices and methods of using them that permit the off-axis placement of the optical fiber used to form the OCT image. Off-axis placement of the fiber would allow the center (core) of the catheter to be used for passing guidewires, additional manipulators, tissue (including cut tissue), drive trains, or the like. However, optical fibers that are positioned off-axis within a catheter may be difficult to manipulate in the formation of a 360° image, since it may be necessary to rotate the entire catheter, rather than just the optical fiber, as is commonly done. Rotation of the entire catheter, including the off-axis optical fiber, relative to a proximate handle or control may result in tangling or binding of the optical fiber at the proximal location. This could ultimately lead to degradation of the image quality and a break in the workflow of the catheter lab environment while the optical fiber is untangled or managed during a surgical procedure.
The devices and systems described herein typically describe catheter-based, off-axis OCT systems that may address many of the needs and problems described above.